Same-sex competition and sexual conflict expressed through witchcraft accusations

There is significant cross-cultural variation in the sex of individuals most likely to be accused of practising witchcraft. Allegations of witchcraft might be a mechanism for nullifying competitors so resources they would have used become available to others. In this case, who is targeted may result from patterns of competition and conflict (same-sex or male–female) within specific relationships, which are determined by broader socio-ecological factors. Here we examine patterns of sex-specific accusations in historic cases from sub-Saharan Africa (N = 423 accusations). Male ‘witches’ formed the greater part of our sample, and were mostly accused by male blood-relatives and nonrelatives, often in connection to disputes over wealth and status. Accusations of women were mainly from kin by marriage, and particularly from husbands and co-wives. The most common outcomes were that the accused was forced to move, or suffered reputational damage. Our results suggest that competition underlies accusations and relationship patterns may determine who is liable to be accused.

The societies in the sample rely mainly on agriculture for subsistence. One society was slightly exceptional for a couple of reasons. The Mbuti speak a Bantu language but are hunter-gatherers rather than agriculturalists and pastoralists. They were not originally part of the Bantu ethno-linguistic group, but probably acquired the language and a low level of witchcraft belief from their Bantu neighbours [4][5][6] . The one case of witchcraft accusation that was recorded from the Mbuti is included in the dataset 7 .

Accusation variables
We coded these variables from ethnographic materials with information on witchcraft cases (see the references below for a full list of sources).

Sex of the accused
This was a straightforward variable to code, as the sex of the accused individual was almost always clear from the text. If it was not clear or not mentioned by the ethnographer, the case was excluded from the analysis. The categories here include situations where the accused was a single individual of either sex, and some instances where a group of men or a group of women was accused. When there were several accused witches, cases were generally coded separately for each individual. Groups of men and groups of women were only included as a single case when there was no distinction between those accused, in terms of their identity or relationship to their accusers or 'victims', such as 'a group of men' or 'some women from the village'.

0) Adult or child 1) Elderly
Many ethnographers only mentioned the age of accused individuals who were particularly old or young, which is why this variable has broad categories. It seems likely these characteristics were recorded because they were notable and could have a bearing on accusations. It was usually apparent when the accused was an adult.
However there may be some cases where ethnographers omitted to record whether an accused 'sorcerer' was particularly elderly or not. This could be slightly more difficult to code than the sex of the accused, as ethnographers sometimes did not view it as relevant, and only gave a name or an initial. Where it was not possible to determine the sex of accusers, it was classified as 'NA'. There were cases where the accused was targeted by a number of accusers of mixed or indeterminate sex, and these have also been included in the 'NA' category.

0) Male/s 1) Female/s
This variable refers to the sex of the purported 'victim' of witchcraft. As above, this was sometimes more difficult to code than the sex of the accused. Where it could not be determined (for example many of those thought to be targeted by sorcery are infants), it was classified as 'NA'. There are also instances where 'victims' are livestock, rather than humans. Such cases were included under 'NA'.

Combining 'victim' and 'accuser' variables
We created variables that combine 'accusers' and 'victims' because the relationship driving an accusation appeared to vary by case.
A variable (not used in analysis) was coded to show whether the accused 'witch's' relationship with the 'victim' was important in an accusation. This was used to select whether the coding for an accuser or a victim was included in the combined variables: The 'victim' is important 0) Yes, the relationship appears to be important in the context of an accusation There are a minority of cases where there was no purported 'victim' of witchcraft, in which case the relationship with accusers was automatically included in the combined variables.

The accuser and the 'victim' are the same
This was the case for 59% of all cases in the dataset. This relationship was included in the combination variables.
A typical example of this form of accusation is given by Beattie 8 from the Nyoro.
Two women, Keziya and Nyenjura, were married to the same man, Kezironi. Keziya had two children, but Nyenjura had none. Keziya became ill, and went to a diviner who confirmed her suspicions that she was the victim of sorcery by Nyenjura. Keziya's condition became worse, and the case ended when she assaulted and killed Nyenjura, for which she was prosecuted in the Protectorate courts.

The 'victim' is incidental
Cases where the victim-accused relationship was categorised as unimportant in terms of the accusation. This included all relationships where the 'victim' was an infant or livestock.
Although the victim can be incidental, they are often connected to the accuser. The sickness of a 'victim' often precipitated an accusation by an accuser who already had a grievance against the accused. In other cases accusations may occur because the accuser is looking for an explanation for the illness or death of their relative.

Competition and conflict between the 'victim' and the accused appears more important than the relationship between the 'witch' and their accusers
Jean La Fontaine provided an example of one such case from Bugisu 9 : Magombe and Zune were age-mates and members of a minor lineage. Magombe lent Zune a cow, and after a lengthy amount of time he asked for it back. He then went to where Zune's cattle were grazing and took the cow back himself. Zune was angry and said 'he would not stand for being treated as though he were an untrustworthy stranger, he a kinsman and an age-mate.' Some time later Magombe was taken ill. His brothers accused Zune of bewitching him. Zune denied any responsibility for Magombe's illness, but eventually conceded to eat food with Magombe (a way of cancelling witchcraft), after which Magombe recovered.
In the main article, we use the variables where the accuser and 'victim' categories are combined. However, we also conducted the analysis where all variables were between the accused and the accuser, as given below in Table S5. As noted in the main text, there are often several concurrent factors involved in a witchcraft accusation. Most cases involve arguments or conflict of some sort. This variable originally had a large number of categories, which were difficult to code in some cases, as it was challenging to reduce situations to a single 'type'. We reduced the categories to those listed above, which has increased the reliability of the code. There were some cases where more than one category remained relevant, which were coded under 'mixed'. We avoided inferring reasons from relationships as much as possible. Although reasons are often stated without ambiguity in ethnographies, they are dependent on the perspective of the ethnographer and their informants, although how much was inferred is likely to vary by case. Some cases are fairly detailed and give the impression that observers were well-informed about the circumstances leading up to an accusation. Others may have been based more on the ethnographers' unique interpretation and therefore less reliable.

Fertility and relationships
This category involves cases where accusations are made because of: • Arguments and jealousy or inferred jealousy (e.g. where an infertile wife has been assumed to be jealous of a fertile wife and is therefore a witch) between co-wives

Interpersonal factors
This category includes: • Cases where the accused was viewed as being difficult, argumentative and uncooperative, or failed to adhere to expected social norms (and this appears to have been their general reputation and pattern of behaviour, rather than having a basis in one-off incidents or a relationship with a particular individual) • Cases where the accused already has a reputation as a 'witch' • Arguments that generally stem from personal matters rather than either issues with categories 0) and 3) • Instances where the 'witch' has a trait or disorder, such as epilepsy, which appears to be why they are

Miscellaneous and Mixed
Miscellaneous includes cases where an accusation seems to stem from a need to explain a death or illness, or an otherwise inexplicable event (such as surviving an attack by a crocodile), rather than because of who the accused is in relation to their accusers or 'victim'.
Mixed cases are those where two or more of the other causes appeared relevant to an accusation. Table S1. Inter-rater reliability measured by percentage agreement and Fleiss' Kappa. Ensuring the reliability of data produced from converting text from ethnographic materials into variables for quantitative analysis is crucial. In order to assess the inter-rater reliability of our data, 3 coders independently reviewed a subsample from our overall dataset (43 cases from 3 societies). We used the irr package in R 10 to calculate a kappa statistic, which penalises levels of agreement that could be due to chance. For this reason, percentage agreements should also be taken into account, as variables which only have a couple of categories (e.g. 'Accused Age) may be penalised heavily in kappa calculations. NA variables are automatically excluded 10 .

Variable
Percentage  We had a number of predictions about socio-ecological factors that may produce higher levels of competition directed at men or women, and subsequently more witchcraft accusations targeting individuals of that sex. These are associated with the question in the main article of why men or women might be more liable to be accused in the context of individual relationships. Some predictions are alternate to one another: for example we predicted that women would be accused more in polygynous societies due to competition for reproductive resources, but it is also the case that polygyny tends to arise in association with differentiation in male resource distribution, which could lead to more accusations of men 11-15 .

Men are more likely to be accused of witchcraft than women in societies with patrilineal and patrilocal social systems
Where descent and inheritance are traced through the male line, related men (particularly fathers, sons and brothers) are more likely to have to compete intensely with one another in order to acquire the resources they need to marry and reproduce 13 . Men rely on their fathers and other members of the patriline to acquire brideprice for marriage 13,14 . The competition for acquisition of status and resources would be even more pronounced among unrelated men, without the mitigating effects of inclusive fitness 16 . The polygynous marriage systems that are common in Bantu societies and others with patrilineality and patrilocality lead to a skew in reproductive success, meaning that male-male competition is likely to be very intense 17 .

More women will be accused of witchcraft in societies where polygynous marriage is common, compared to those where it is limited
As wives in polygynous marriages compete with each other for reproductive resources, there will be more witchcraft accusations targeting women in societies where there are higher levels of polygyny, compared to those where there are lower levels 11,12,15,18 .

More men than women will be accused of witchcraft in societies with greater social stratification
Where resources are unevenly distributed and there is greater variation in male social status, there will be greater competition between men to acquire the resources they need to maximise their reproductive success 17 .
Therefore nullifying competitors for resources and mates through the means of witchcraft accusations may be an adaptive strategy.

Society-level variables
All of the society-level variables used here, and included in the analysis (Tables S2 and S4; Fig. S1) are from Murdock's Ethnographic Atlas 2,3 . We re-coded the variables from original EA categories as shown below, with original category numbers shown in brackets.

DISCUSSION: SOCIETY-LEVEL RESULTS
None of the society-level variables significantly affected whether witchcraft accusations targeted men or women in Bantu societies. As they were all included in the top models in the selection process, they all had some effect on the outcome variable, but it was much smaller than that of individual-level variables. This does not support our predictions, previous historical and anthropological perspectives suggesting that society traits may lead to accusations being directed at a particular sex 21,22 , or what might be predicted from evolutionary theory if witchcraft accusations are a mechanism for harming competitors 13,15,17 .
There are several possible explanations for the lack of effect of society-level variables on the sex of individuals accused of witchcraft. The first is sample size: although this is the largest sample of its kind that we are aware of, with more detailed data, society-level differences might be more pronounced, but this was inevitably constrained by the availability of ethnographic resources. It may also be the case that high levels of competition towards members of one sex in a society is associated with high levels of competition towards the other sex, which therefore almost cancel each other out. In patrilineal and patrilocal societies, both male-male and female-female competition is high, and there may be more inter-sexual conflict. Male-male competition is intense due to competition for mates and the resources to needed acquire them 13 . Female-female competition is strong when women are competing to obtain resources for offspring, as in societies with polygynous marriage 11,12 . Male control of females may be more pronounced where men have higher control of resources and there is a greater skew in male reproductive success 23 . The corresponding reduction in female social status could have two possible effects: it might render women less able to defend themselves and more vulnerable to being accused, but also reduced prestige and access to resources might mean accusations would be less likely to target them in the first place 24 . Because these society-level patterns of accusation may obscure and counter each other, individual-level relationships might in some ways be better at predicting competition and accusations.
It is also the case that there is limited variation between our sample of societies in some aspects of their socioecology. The majority of Bantu and Bantoid societies in the sample are polygynous with male-biased inheritance systems (even among matrilineal groups 2,3 ), and these may be the crucial factors in determining patterns of accusation. The ICC result provides some support for this, as it suggests there was only a moderate amount of variation (15%) between societies and more substantial variation within societies in terms of the proportion of accusations directed at men and women.
Other researchers such as the historian Ronald Hutton have emphasised that finding patterns in the distribution of witchcraft beliefs proves extremely elusive, as they appear to bear little association with social organisation, environment or culture 25 . Hutton's conclusion was reached after an extensive worldwide survey of over 300 societies' witchcraft beliefs, although this was qualitative, and so may not be as precise in identifying crosscultural patterns as quantitative methods. Hutton was examining the existence and intensity of witchcraft beliefs, rather than why accusations targeted individuals of a particular sex. But as witchcraft accusations and beliefs are highly flexible cultural traits, that can easily be adapted to a variety of circumstances, this may further account for why society-level patterns are not more pronounced.

Sensitivity analyses for ethnographer bias
Many ethnographers did not document witchcraft accusations systematically. Instead they mentioned cases they regarded as typical of the society at that time, or ones that illustrated a particular point, or simply those that caught their attention. Ethnographers were more likely to report cases that were memorable or dramatic. In order to examine how much bias on the part of ethnographers may have impacted our main results, we conducted analyses using only societies where a) ethnographers attempted to systematically document all cases they heard about during a period of fieldwork and b) where ethnographers were female.

5A. Systematically documented cases
Descriptive statistics

5B. Cases documented by female ethnographers
In our sample more men than women were accused of witchcraft. This could potentially represent a bias on the part of mostly male ethnographers, who were more interested in accusations of men (although it could also be argued that European ethnographers would bring the preconception that 'witches' were female). We used a further sub-sample of all cases where an ethnographer was known to be female (two documents were produced by husband-and-wife ethnographers working together) and analysed them to see if this made difference to our results.
Descriptive statistics Table S10. Counts of men and women accused of witchcraft when ethnographers were female.

63 147
Society N = 11 (Bagisu, Banen, Bemba, Chagga, Ganda, Gikuyu, Lovedu, Nyakyusa, Nyanja, Pondo, Tiv) Table S11. The results given below are from a multilevel logistic regression using individual-level variables to predict the odds of the accused 'witch' being male or female. This was calculated using the sub-sample of cases with female ethnographers. Significance levels: *** p < 0.001, ** p <0.01, *p <0.05. In these sub-samples, the result that an accusation from affinal kin is more likely to target a woman than a man remains. Similarly, accusations from unrelated individuals have a higher odds of targeting men than women, as do accusations from blood relatives. Accusations directed at elderly people were no longer more likely to target women than men. However, this variable had to be excluded from the sub-sample of systematically recorded case studies as there was only one accusation in that category. The number of elderly individuals remained small in the sample of accusations recorded by female ethnographers (N = 11).

Variable
We have used these subsamples to investigate the extent that ethnographer bias may have influenced our results.
The finding that accusations from affinal kin are more likely to be directed at women than men, and that accusations from non-relatives and blood relatives are more likely to target men, remains similar. The greater weight of accusations in the cases used for sensitivity analyses were directed at men. This provides some assurance that our original results are not excessively biased by anecdotal sampling or focus on cases where the accused was male on the part of male ethnographers. The variable relating to the situation ethnographers identified as causing accusations no longer produces a statistically significant result.
It should be noted that these sensitivity analyses do not have satisfactory statistical power due to reduced sample sizes 26 and can therefore only be regarded as an exploratory investigation of these factors.

ETHNOGRAPHY META-DATA
To further examine how likely our results were to be affected by bias, we collected meta-data on aspects of how ethnographies were produced: how long the ethnographer conducted fieldwork for, how fluently they spoke the language, when the ethnography was published, and the ethnographer's nationality. We examined how these factors and the sex of the ethnographer were associated with the main variables in our analysis. These were coded for individual ethnographies, rather than ethnographers. This was intended to be an exploratory analysis to identify any particular trends as well as the fact that including meta-data on ethnographic materials is good practice when in this type of study 27 . But we did not have hypotheses about how bias on the part of ethnographers might lead to our results, such as whether ethnographers would be more likely to record or observe accusations of women by affinal kin than reflected the actual . In terms of the variables, we predicted that the variable linking witchcraft accusations with situations might be more unreliable, as ethnographers were attempting to infer causal associations or relying on inferences from their informants to do so. With the sex of the accused and others involved (where it was documented) and relationships, there would perhaps be a reduced margin of error as such details are generally less open to as they are mostly readily observable. But as discussed in the main text there is also the potential for selection biases in what ethnographers chose to record.

Variables
Publication date The date an ethnography was produced and published might have an effect on how prevalent particular cultural biases were prevalent at the time, and how trained ethnographers were at to be aware of their own biases 27 . Publication dates were grouped into time frames: Pre-1920Pre- , 1920Pre- -1939Pre- , 1940Pre- -1959Pre- and 1960 onwards. Ethnographer sex As above, this was grouped into whether the ethnography was written by a man or whether it was written by a woman or a woman and a man.

Methods
We collected the data mainly from the ethnographic records used in the main study. Details were generally available in the Preface and Introduction of the documents. For some, we accessed alternative online sources of information such as alumni profiles or journal articles that provided meta-data on the production of ethnographies.

Statistical analysis
Variables with multinomial categories (Situation and Relationship) were converted to dummy variables so we could examine focal categories. We intended to perform multilevel analyses with the ethnographies as random effects, but as some texts only produced one case study this led to issues with model singularity. We therefore ran logistic regressions to compare how attributes of ethnographies were associated with outcome categories for all our main variables.
Variables were checked for collinearity using car 28 . As some variables were collinear we excluded one from particular models when the VIFs were over 5. For example 'Ethnographer type' was often collinear with publication date, as earlier ethnographies tended to be written by missionaries and colonial officials.    Tables S13-16). This is likely to be due to there being a relatively low number of observations for that category, such as where ethnographers were colonial officials (N=16) or where they did not speak the language at all (N=5).

Results
Some attributes of ethnographies were significantly correlated with categories in the variables.
Only having a partial grasp of the language, as compared to being fluent or near-fluent, had significant associations with particular categories across the variable indicating the sex of the accused, the relationship driving the accusation, and the situation thought to have caused an accusation. Having a partial grasp of the language indicated that ethnographers were more likely to record accusations of women.
The 'situation' variable had the highest number of significant associations. Some of these related to the date a work was published. Publications after 1960 were significantly more likely to identify the causes of accusations as mixed or miscellaneous, rather than associating them with fertility and relationships, interpersonal reasons or wealth and status. Ethnographers whose studies were published between 1940-1959 were more likely to code interpersonal factors as the cause of accusations than any of the other categories and less likely to attribute them to miscellaneous or mixed causes.
Male ethnographers were less likely to document the relationship in an accusation as between unrelated individuals than any other category and more likely to note those between related individuals than within the unrelated or affinal categories.

Discussion
Our exploratory study of the correlations between the attributes of ethnographic records and the categories identified by ethnographers in our outcome variables indicates that there are some significant correlations. The situation connected to a witchcraft accusation had a higher number of significant associations than other variables used in our main study. This may indicate that ethnographer observations of this variable are more likely to be unreliable and should be treated with caution.
It is somewhat uncertain how much the correlations in this analysis reflect true trends in bias among ethnographers. Some results have clearer possible explanations than others: a partial grasp of the language and greater reliance on interpreters might mean that ethnographers tended to rely on their preconceptions. For example European ethnographers may have expected witches to be female and not been able to interpret information about accusations of men. Other correlations, such as male ethnographers more frequently recording the significant relationship in an accusation as between relatives and less frequently as between unrelated individuals, do not have obvious explanations in terms of ethnographer bias and may be spurious.
Bias or random error may affect ethnographers' documented observations, and this is a significant limitation when conducting research using ethnographic records.